如何编写匹配无前后空格字符的正则表达式？附应用示例

Hey there! Let's tackle your problem of matching characters that don't have spaces before or after them, and getting that desired string transformation you mentioned.

First, let's clarify the target: we want single non-space characters that are sandwiched between other non-space characters (no spaces immediately before or after them). That's exactly the "a" in "sample" from your example—since it sits between "s" and "m", no spaces surround it.

The Regex #

Here's a regex that does the job (works in most modern regex engines like JavaScript, Python 3.6+, Java, etc.):

(?<=\S)(\S)(?=\S)

Regex Breakdown #

• (?<=\S): A positive lookbehind assertion that checks the character right before our target is a non-space character (\S stands for any non-whitespace character).
• (\S): The capture group that grabs the actual character we want (a single non-space character).
• (?=\S): A positive lookahead assertion ensuring the character immediately after our target is also a non-space character.

If you're working with an older regex engine that doesn't support lookbehinds (like pre-3.6 Python), use this alternative approach with capture groups:

(\S)(\S)(\S)

Replace matches with $1 $2 $3—this takes three consecutive non-space characters and adds spaces around the middle one, achieving the same end result for your use case.

Example Usage for Your Scenario #

Take your original string:
This is a sample String

Using the first regex, replace the captured character with $1 (adding spaces before and after it). Running this replacement gives you:
This is a s a mple String

Code Example (JavaScript) #

const originalStr = "This is a sample String";
const targetRegex = /(?<=\S)(\S)(?=\S)/g;
const transformedStr = originalStr.replace(targetRegex, ' $1 ');
console.log(transformedStr); // Outputs: "This is a s a mple String"

Quick Notes #

• This regex targets non-space characters only (since your example focuses on "a", which isn't a space). If you ever need to include spaces (though that doesn't fit your scenario), swap \S with .—just be mindful of newlines if you don't want those included.
• It won't match single characters at the start or end of the string, since those don't have both a preceding and following character to be "sandwiched" between.

内容的提问来源于stack exchange，提问作者Mohd. Shahid